While making reservations for our five week trip out west, I ran across one website for a campground that actually stated that all the campgrounds in Las Vegas had 10 year rules, and to not even try to get into their grounds if your rig was more than 10 years old because they would check the serial number plate inside the rig.
This turned out to be somewhat of a joke because I found another park, also with the 10 year rule, that was happy to take my 11 year old coach after seeing a picture of it. The manager told me they had that rule stated on their website only as a fall back if needed to keep out the homemade rigs or those that looked like they were falling apart. And, after further research, I found several parks in that area that weren't concerned about age at all.
The best thing to do is to call the parks and ask. Some are strict about the rule because they want to give an air of being better (tongue in cheek for "snob"), while others are quite lenient and only post that requirement to avoid the really bad looking units. My guess is that half or more of the parks that post that restriction will bend their rules if they want your business.